Tentacle structure and filter-feeding in Crisia eburnea and other cyclostomatous bryozoans, with a review of upstream-collecting mechanisms

Abstract

ABSTRACT-The upstream-collecting filter-feeding mechanisms occurring in many aquatic organisms are not adequately descnbed. Tentacles of Crisia eburnea and other cyclostome bryozoans, with only lateral and laterofrontal ciliary bands, are among the least complicated upstream-collecting systems among metazoans. SEM and TEM revealed that the tentacles have 2 rows of very closely set lateral cilia and 1 row of stiff laterofrontal cilia on each side. The shape of the basal membrane and the longitudinal muscles indicate that the tentacles are specialized for flicking movements directed towards the centre of the tentacle crown. Video observations of C. eburnea feeding on Rhodomonas cells showed charactenstic velocity gradients around the tentacle crown. Particles in the central current galn the highest velocity at the entrance to the tentacle crown from where the speed decreases to zero in front of the mouth. Usually the path of a particle deviates from the downward course to a more outwards directed course (between the tentacies), where they may be irclppeci by iile iiiiel iarnied by the stiff Ieteiofron:~! cilia; the tentacle then makes a flick that brings the particle into the central current and further down towards the mouth. A survey of the literature shows that a similar mechanical filter mechanism occurs also in gymnolaemate bryozoans and their cyphonautes larvae, but that the particle-collecting mechanism of larvae and adults of phoronids, brachiopods, pterobranchs, and enteropneusts is different. The differences in structure and functlon between the tentacles of ectoprocts and those of phoronids, brachiopods and pterobranchs support the idea that the 2 types of tentacle crowns are not homologous.

Molecular phylogenetics of the Metazoan clade Lophotrochozoa

“…However, identification of a Post2 class Hox gene from the bryozoan Bugula 94 turrita, as described in Chapter 4, provides strong evidence for a close relationship between bryozoans and other lophotrochozoans, upholding the phylogenetic validity of the clade as it was initially defined. These finding are consistent with recent work questioning the homology of the bryozoan tentacles to the lophophore of brachiopods and phoronids (Nielsen and Riisgård, 1998). It is therefore suggested that, to avoid the suggestion of homology, the term "lophophore" be reserved only for the tentacular feeding structure of phronids and brachiopods, and not that of bryozoans.…”

Minireview: Ciliary filter feeding and bio‐fluid mechanics—present understanding and unsolved problems

“…In most bryozoans, three types of ciliary rows may be found on the tentacles: lateral, frontal, and laterofrontal. Lateral cilia produce the feeding currents of the lophophore (Gordon et al 1987;Nielsen 1987. When the through currents pass between the tentacles, they form a relatively strong core current directed straight down the lophophore to the mouth.…”

“…The present description of the catch-up principle further substantiates the hypothesis proposed by Strathmann and Leise (1979), who found that suspended particles are concentrated when they are ''overtaken'' by cilia in their effective stroke and ''pushed faster than the water.'' Ciliary upstream-collecting animals possess ciliary bands that divert suspended food particles from the main water current and concentrate them on the upstream side of the band (Bullivant 1968a;Strathmann 1971Strathmann , 1973Strathmann and Bonar 1976;Gilmour 1979Gilmour , 1982Gilmour , 1985Nielsen 1987Hart 1990Hart , 1991Hart and Strathmann 1994; for review, see Nielsen and Riisgård 1998). The capture mechanism is correlated with the presence of a single band of cilia, which appears to be responsible for both the water transport and particle retention.…”

“…Thomassen and Riisgård (1995) Reiswig (1974) Reiswig (1974) Reiswig (1974) Reiswig (1974) Riisgård (1988a) Petersen et al (1995) Jørgensen (1955 pumps are found in, for example, bivalves, ascidians, and bryozoans. Geometries and motions of cilia in many pumps are known (e.g., Sleigh 1974; Brennen and Winet 1977;Silvester and Sleigh 1984;Nielsen 1987), and the pumping mechanism is understood in principle. A system characteristic is the total pressure drop (⌬H s ) as a function of flow through the system.…”

“…According to the trochaea theory (Nielsen 1987, the upstream particle-collecting mechanism is restricted to the deuterostomes-adult pteobranchs, larval and adult phoronids, and brachiopods, where the feeding structures of the adults are derived from those of the larvae-and larvae of echinoderms and enteropneusts. Sensory laterofrontal cilia (although not electrophysiologically confirmed) are found in phoronids, pterobranchs, and brachiopods, but their presence has not been reported in echinoderms and enteropneusts (Gilmour 1978(Gilmour , 1979(Gilmour , 1981Nielsen 1987). The function of the laterofrontal cilia remains unknown, but a role in control of the lateral cilia may be suggested.…”